This invention relates to novel hydroxyethyl hydroxypropyl cellulose ether thickeners for aqueous coating compositions.
Water-soluble cellulose ethers have long been employed in aqueous coating compositions as thickeners and protective colloids. Ideally, cellulose ethers employed in said aqueous coating compositions must provide good flow and brush out, good color development and high resistance to bacterial degradation. Moreover, it is highly preferred that the cellulose ethers have a haze point such that the cellulose ethers remain soluble at the high temperatures (i.e., 50.degree.-90.degree. C.) produced in the commercial formulation of said aqueous coating compositions yet have a gel point below 100.degree. C. so that the cellulose ethers can be purified inexpensively by using aqueous media. Cellulose ethers previously employed as thickeners in latex coating compositions generally do not exhibit the desired combination of good color development, high resistance to bacterial degradation and high haze point.
For example, hydroxyethyl cellulose ethers exhibit good color development but low resistance to bacterial degradation. See, for example, Lindenfors, Acta Chemica Scand. 16,111 (1962). Hydroxypropyl methylcellulose ethers exhibit improved resistance to bacterial degradation yet exhibit poor color development, i.e., they cause significant agglomeration or flocculation of the pigments, thereby causing the loss of pigment effectiveness. Hydroxyethyl hydroxypropyl methylcellulose ethers often have a haze point which is too low to permit normal formulation of the coating composition, and are relatively expensive to produce due to the formation of undesirable by-products.
Thus, a cellulose ether which combines the desired characteristics of good color development, high l resistance to bacterial degradation and high haze point is highly desirable for use in aqueous coating compositions.